1. Field of the Invention
The present invention relates to vehicle steering systems in which both front and rear road wheels are steerable.
2. Description of the Prior Art
FIG. 1 of the drawings shows, by way of example, a system for controlling steering of rear road wheels in dependence upon steering of front road wheels, which system is disclosed in U.S. Pat. No. 4,440,254 issued on Apr. 3, 1984 in the name of Shibahata et al and assigned to the same assignee as the subject application.
In FIG. 1, the reference numeral 10 indicates a sub-frame of a semi-trailing arm suspension for rear road wheels 12 and 14. The sub-frame 10 is bolted to a differential gear 16 which is in turn mounted on a vehicle body (not shown) through a pivot pin 18 and an elastomeric insulator 20, so that the sub-frame 10 is rotatable about the pivot pin 18 to steer the rear road wheels 12 and 14. The reference numerals 22 and 24 indicate semi-trailing arms and rear drive axles, respectively.
The sub-frame 10 is also mounted at the opposed ends thereof on the vehicle body through chambered elastomeric insulators 26 and 28. More specifically, each elastomeric insulator 26 or 28 has a cylindrical configuration and is mounted at the outer periphery thereof on the vehicle body and at the central part thereof on the sub-frame 10 through a pin 30 or 32. The elastomeric insulator 26 or 28 is formed with a pair of diametrically opposed variable volume chambers 26a and 26b or 28a and 28b. The chambers 26a and 26b are respectively paired with the chambers 28b and 28a, and the pair of chambers 26a and 28b or 26b and 28a are fluidly connected through conduits 34 or 36 to a directional control valve 38. The directional control valve 38 is in turn fluidly connected through conduits 40 and 42 to a power steering servo and control unit 44 consisting of a power cylinder 46 and a control valve 48. The power steering servo and control unit 44 is for steering front road wheels (not shown) and fluidly connected to a pump 50 and a reservoir 52. The directional control valve 38 is operative to take two valve positions for controlling connections between the conduits 34, 36, 40 and 42 in dependence upon a signal derived from a vehicle speed sensor 54. The reference numeral 56 indicates an amplifier interposed between the directional control valve 38 and the vehicle speed sensor 54.
With the above arrangement, when a steering wheel (not shown) is turned to the left (counterclockwise rotation) during low speed running of a vehicle, the hydraulic fluid pressure in the conduit 40 rises while the conduit 42 is communicated with the reservoir 52 to allow the pressure in the conduits 40 to fall to the atmospheric pressure, thus making it possible to turn the front road wheels to the left under the assist of the power cylinder 46.
During the low speed running of the vehicle, the conduit 40 is communicated with the chambers 26a and 28b through the directional control valve 38 and the conduit 34 as illustrated, thus causing the hydraulic fluid pressure in the chambers 26a and 28b to rise. On the other hand, the chambers 26b and 28a are communicated through the conduit 36, the directional control valve 38 and the conduit 42 with the reservoir 52 to allow the pressure in the chambers 26b and 28a to fall to the atmospheric pressure. The pressure changes in the chambers 26a and 28b cause the sub-frame 10 to rotate about the pivot pin 18 in the clockwise direction in the drawing, thereby turning the rear road wheels 12 and 14 to the right, i.e., to the direction opposite to the direction of turning of the front road wheels.
When the steering wheel is turned to the right during low speed running of the vehicle, the front road wheels are steered to the right, while the rear road wheels are steered to the left.
In the above manner, it becomes possible for the vehicle to make turns of relatively small radii.
Upon high speed running of the vehicle, a signal derived from the vehicle speed sensor 54 is amplified by the amplifier 56 and applied to the directional control valve 38. The directional control valve 38 thus assumes a position other than illustrated in FIG. 1 and connects the conduit 40 to the conduit 36 while the conduit 42 to the conduit 34. In this case, when the steering wheel is turned to the left, for instance, the pressure changes in the chambers 26a and 28b cause the sub-frame 10 to rotate in the counterclockwise direction in the drawing, causing the rear road wheels 12 and 14 to turn to the left, i.e., in the same direction as that of the front road wheels.
In the above described prior art steering system the rear road wheels are turned at any time when the front road wheels are turned, irrespective of the magnitude of a torque or effort with which the steering wheel is rotated.
The prior art steering system has a disadvantage that the rear road wheels tend to oscillate to-and-fro to cause the tail of the vehicle to oscillate sideways during low speed running of the vehicle, resulting in the lowered stability of the vehicle.
Another disadvantage is that the rear road wheels tend to protrude out of the fenders during low speed running of the vehicle, resulting in the possibility that the road wheels strike against something built or constructed.
A further disadvantage is that the vehicle tends to effect a tack-in phenomenon, i.e., tends to make turn of a smaller radius when suddenly decelerated during high speed cornering since the direction to which the rear road wheels are steered during low speed running of the vehicle is reverse to the direction to which they are steered during high speed running, though responding to the same steering of the front road wheels.